Nearly 20% of people are keeping a savings, credit-card or checking account hidden from their live-in partner, according to a survey released this week by CreditCards.com. The company polled 1,000 adults, including 636 who are currently married or living with a partner.

Millennials (ages 18 to 37) are twice as likely to say they’re hiding a bank or credit-card account from their partner (28% versus 15%). Other more common offenders are people in the South (22%) and West (21%), compared to those in the Northeast (16%) and Midwest (12%).

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More than half (55%) say keeping a secret account is as bad as actually cheating on someone, and 1 in 5 say it’s worse than physical infidelity. Older people with higher incomes and education levels were more likely to believe that physical cheating is worse.

The good (and bad) news: Americans who live with their partner are generally confident about their own personal money management skills: 44% percent believe they are a better money manager than their spouse/partner, and just 12% think they’re worse.

Rossman warned against over-confidence, citing statistics about how woefully under-saved many Americans are for emergencies, retirement and even college. “If you don’t know where your money is or where it’s going, those already lofty financial goals will become even harder to reach,” he said.

More than 77% of people consider credit-card debt an unattractive trait in a mate, according to a recent survey released by personal-finance site Finder.com. On average, people say $11,525 in credit-card debt is enough of a red flag to swipe left or walk away from a potential partner.

Payday loans, which can have astronomical interest rates as high as 400%, are the second most unacceptable forms of debt for daters after credit-card debt. As such, it only takes a payday loan of $1,830 to turn off a would-be mate.

Co-mingling finances — or at least leaving your credit card lying around — can also be problematic. This woman recently wrote into MarketWatch’s Moneyist advice column that her husband racked up $7,000 in credit-card debt on her credit cards. “I only make $12 per hour,” she said.

But some couples don’t talk about their respective purchases, and rely on old-fashioned trust instead — and do so successfully. Michael Hayes, 60, an architect, and Jamie Brickhouse, 50, a writer, told MarketWatch that the secret to their finances is that they don’t talk about them.

So what does this New York City couple fight about when they fight about money? “We don’t because we’ve never had a joint checking account,” Hayes said. “He’s been pretty reliable. If he did get himself into a situation where he was carrying short-term debt, he was able to pay it off.”

“Neither one of us talks about the bank account of the other one,” Brickhouse added. “When I started on my own, he would pick up the slack. He would never say, ‘You spend $200 on lunch or Bed, Bath and Beyond? What’s that about?’” The couple has been together nearly 30 years.

Love & Money is a MarketWatch series looking at how our relationship with money impacts our relationships with significant others, friends and family.